There are no half measures for heavy smokers wanting to minimise the risk that their habit will lead to their early death, research suggests.
Tobacco is linked to a range of health problems
Scientists found no evidence that heavy smokers who halve their daily cigarette intake cut their premature death risk.
The long-term Norwegian study, of more than 51,000 men and women aged between 20 and 34, found stubbing out was the only way to cut the risk.
The study is published in the journal Tobacco Control.
The participants were assessed for cardiovascular risk factors at the start of the study, and then monitored for an average period of more than 20 years.
They were classified into various groups, including non-smokers, moderate smokers (up to 14 cigarettes a day), and reducers, who smoked more than 15 cigarettes a day at the start of the study, but who had cut back by more than half by the second check.
Men who had cut back had slightly lower death rates from all causes than heavy smokers during the first 15 years.
However, after that death rates were comparable.
Women who cut back actually had higher death rates from all causes combined than heavy smokers - although the researchers said this could simply be a chance finding.
Light smoking impact
Researcher Dr Kjell Bjartveit said there was evidence to show smoking just one to four cigarettes a day increased the risk of dying from heart disease dramatically.
He said: "It is widespread to offer smokers a last resort: 'If you are unable to quit, cut down'.
"In our opinion, this advice may offer people false expectations. There is only one safe way out: To quit smoking entirely."
Amanda Sandford, of the anti-smoking charity Action on Smoking and Health, said: "This study reinforces the evidence that simply cutting down does not alter the risk of premature death from smoking.
"However, there is good evidence to show that cutting down while using nicotine replacement therapy (NRT) significantly increases the chances of successfully quitting smoking.
"For smokers who are not yet ready to quit but want to cut down, we recommend that they use NRT, otherwise there is a danger that their cigarette consumption will quickly return to previous levels."
Simon Clark, director of the smokers' lobby group Forest, said it was important to take into consideration the fact that many smokers tended to lead a generally unhealthy lifestyle, eating a poor diet, and not taking exercise.
He said: "Unfortunately, campaigners prefer to emphasise the 'quit or die' message, which is a gross exaggeration, but if you are a heavy smoker it is surely better to reduce consumption, combining that with a healthy diet and plenty of exercise, than not cut down at all?"
A separate study in the same journal found that pregnant smokers may "programme" their children to take up the habit.
The Australian study of more than 3,000 mothers compared the smoking patterns of their children when they reached the age of 21.
Children whose mothers had smoked while pregnant were almost three times as likely to start smoking regularly at or before the age of 14, and around twice as likely to start smoking after this age, than those whose mothers were non-smokers.
Smoking patterns among children whose mothers stopped smoking while pregnant, but then resumed the habit, were similar to those whose mothers had never smoked.